Fault code P0118 – a step-by-step guide to diagnosing and fixing it

DTC P0118 is triggered by the control unit when the signal from the engine coolant temperature (ECT) sensor of the car indicates that the voltage input from the ECT circuit is too high. The sensor is a thermistor that is used to monitor temperature changes in the coolant system. The device’s electrical resistance fluctuates based on the temperature, causing changes in the voltage signal. The code is usually registered when the voltage is around 4.7 V - 4.91 V for a set amount of time, or the resistance is out of the normal range.

• A short in the voltage supply wire connected to the control module and ECT device. This can occur due to thermal or mechanical damage, such as a fraying or melted insulation sheath.
• Oxidised electrical contacts, resulting in a bad connection. Oxidation can occur if the parts are exposed to dirt and moisture.
• A failing thermistor, caused by engine components overheating.
• A contaminated sensor. The cause of contamination is often a rusty coolant system, old antifreeze, or dirt contamination in the coolant expansion tank of the car.
• Air pockets in the coolant lines. This slows down the temperature regulation process of the fluid.
• Low coolant levels, affecting the sensor readings.

• Higher fuel consumption
• Activation of the check engine light
• Starting problems
• Black smoke from the car's exhaust system parts
• A rough idle
• Engine misfires
• Poor engine performance

• Use an OBD2 scan tool to check for related DTCs, and read through the freeze frame data from when the DTC was set. This information should help you to identify the cause of the fault and obtain a proper diagnosis.
• Inspect the wires and connectors for signs of damage or significant corrosion. You may need to disconnect the ECT sensor and check the circuit wires for wear or connection problems. Replace damaged components and remove traces of corrosion from the electrical contacts using a special cleaning agent.
• Examine the cooling system to see whether the coolant levels are sufficient, there are air pockets, or the liquid is contaminated. It may be necessary to bleed the system and change the antifreeze for car.
• You may need to measure the resistance of the ECT circuit using a multimeter. Check the vehicle manual for information on detaching the device before proceeding. If the circuit is functional, the reading should be around 2000 Ohms at room temperature. This value should decrease after applying heat (e.g. using a hairdryer). If the readings were abnormal, replace the sensor.